A known switching power source of self-induced flyback type such as RCC (ringing choke converter) is defective in that it increases on-off switching frequency of a switching element when an electric load becomes light because the on-period of the switching element becomes shorter with the shorter period of producing flyback voltage on a secondary winding of a transformer. A typical on-off switching frequency or oscillation frequency of switching element generally ranges from 30 to 70 kHz under the maximum load and from 200 to 400 kHz under the minimum load. Thus, with the lighter load, the number of times the switching element is turned on and off is increased, resulting in augmentation of switching loss and reduction of conversion efficiency during the light load period. Accordingly, even a switching power source of 85% conversion efficiency under the maximum load may sometimes reduce the conversion efficiency equal to or less than 10% under the minimum load.
A typical switching power source of PWM (pulse width modulation) flyback type does not change the switching loss of switching element because oscillation frequency is constant either under the minimum load such as stand-by state or under the maximum load such as normal state. However, as electric loss other than switching loss is decreased during the light load period, switching loss occupies a major proportion, reducing the conversion efficiency.
To overcome the foregoing defects, Japanese Patent Disclosure No. 9-140128 demonstrates a switching power source which, as shown in FIG. 26, comprises a microcomputer 108 provided on a secondary side of a transformer 106 for detecting or controlling operation of a device; and a delivery circuit 109 for transporting control signals from microcomputer 108 to a primary side of transformer 106 to control oscillation frequency of the power source by microcomputer 108 during the stand-by mode. A switch element 101 is connected to a primary winding of transformer 106 to control electric current flow through the primary winding. A drive circuit 102 produces drive signals to switch element 101 which is turned on when electric voltage applied on a control terminal of switch element 101 reaches a threshold level. A drive controller 103 serves to control on-time of switch element 101 to stabilize the output voltage from secondary side. A secondary rectifying and smoothing circuit 104 is connected to a secondary winding of transformer 106, and a primary rectifying and smoothing circuit 105 is connected to a primary auxiliary winding of transformer 106. Transformer 106 functions to electrically insulate between primary and secondary sides, and simultaneously, forms an electromagnetic coupling to convert a primary input electric voltage into a desired secondary output voltage. A detector 107 picks out a secondary output voltage generated from secondary rectifying and smoothing circuit 104. This switching power source is advantageous because it reduces switching loss in the stand-by mode or during the light load period, thereby resulting in improved conversion efficiency, however disadvantageous because it requires increased number of required components that causes rise in cost of manufacture. Also, it is actually impossible to apply the switching power source to AC adaptors for small electronic devices such as mobile phones or personal handy phone systems or portable personal computers because the power source involves a large-scale addresser such as a microcomputer.
In the meantime, to detect electrical load condition on the secondary side, the load current has to be detected on the second side and then transmitted to the primary side. Alternatively, a microcomputer has to be provided in the secondary side to deliver command signals from such a microcomputer to the primary side. However, in any event, these arrangements have a drawback of increase in number of required components for assembly. Accordingly, it would be necessary for the power source to accurately discern the secondary load condition directly from the primary side in order to minimize number of required components. Techniques for appreciating the secondary load condition on the primary side include measurements of: switching current flow through a switching element; voltage feedback signals from secondary side; and time period of flyback voltage generated through a winding of transformer. The first technique for measuring switching current flow through switching element is generally realized in many cases as an over-current protector (OCP) which may comprise a resistor for detecting electric current therethrough and a comparator connected to the resistor. This technique, however, unfavorably may cause capacitative short-circuit over-current to flow through the switching element at the moment of turning it on as shown in FIG. 27 due to a parasitic capacitance formed by an inherent structure in the switching element; a snubber circuit (such as a capacitor) connected between two electrodes of the switching element for reduction of noise or another snubber circuit connected between windings of transformer to reduce noise and protect the switching element. The capacitative short-circuit over-current disturbs the accurate detection of secondary load condition because the over-current cannot be determined only by the secondary load condition, and moreover, a peak value of the over-current may be elevated over a peak value of a secondary load current during the light load period. For that reason, without accomplishing any purpose for detecting the secondary load condition, namely, light or heavy load condition, the foregoing current detecting resistor is generally used as a protective circuit against over-current for restricting excessive switching current in case of preventing some malfunction of the switching element (for example, over-load condition by a damaged secondary circuit or uncontrolled condition by a damaged control system). Accordingly, such a prior art switching power source makes it very difficult to exactly detect the secondary load condition on the primary side with minimum number of required components in order to select an optimal oscillation operation based on the detection result on the primary side and thereby improve the conversion efficiency of the power source.
Hence, an object of the present invention is to provide a switching power source capable of exactly detecting a secondary load condition on a primary side of a transformer for improvement of the conversion efficiency.